1. Field of the Invention
The present invention relates to mounting structures for cutters, and more particularly to a tapered mounting structure for mounting a cutter onto a rotatably driven holder.
2. Description of the Prior Art
Cutters (also sometimes referred to as "side cutters") used in the machine tool industry have a circular cross-section, the outer periphery of which being provided with cutting teeth of a predetermined structure. In order for the cutter to rotate so that the cutting teeth engage rotatably with a workpiece, the cutter is provided with a center bore for receiving a shank of a holder. In this regard, the holder is mounted on machinery for providing rotation of the holder about the centerline of the shank.
The state of the art practice for mounting cutters is depicted in FIG. 1. A holder 10 in the form of a shell mill adapter is provided, which would connect to some conventional rotary motion generating machine. The holder 10 includes a holder hub 12 defined at one end by a shoulder 14. A shank 16 projects from the shoulder 14, and includes an axially disposed threaded bore 18. A cutter 20 includes a center bore 22 which has been slipped onto the shank 16. A pair of keys 24 are provided on opposite sides of the shoulder 14, and which are partly resident in slots 26. The cutter hub 28 includes slots 30 for partly receiving the keys 24. A hold-down screw 32 having a head 32a threadably engages with the threaded bore 18 until the cutter 20 is firmly held between the shoulder 14 and the head 32a.
While the mounting structure indicated in FIG. 1 provides a reasonably good mounting for the cutter, there are several severe disadvantages present.
One problem has to do with concentricity. Any tolerance between the inside diameter of the center bore 22 and the outside diameter of the shank 16 will almost certainly result in an asymmetric mounting of the cutter with respect to the centerline C of the shank. Consequently, the periphery of the cutter 20, which is circular, will not concentrically rotate about the centerline C. This untoward situation will result in uneven tooth engagement at the workpiece. As an example, consider a machining operation having a 0.002 inch cut (chip load) per cutting tooth. If the centering of the center bore 22 on the shank 16 is off by 0.002 inch, intermittent tooth engagement with the workpiece can occur, resulting in not only uneven tooth wear, but rough surface finish on the workpiece. This effect is magnified for cutters of increasing diameter. And, this problem cannot be conventionally eliminated because some tolerance is inevitably needed in order to slip the center bore 22 onto the shank 16.
Another problem has to do with perpendicularity. The tolerance between the center bore 22 and the shank 16 permits the cutter to be mounted in a non-perpendicular relation to the centerline C. While the shoulder 14 and the head 32a may mutually serve to align the cutter 20, a small variation in alignment between the head, the shoulder and the cutter hub 28 will result in a non-perpendicular alignment of the cutter with respect to the centerline C. As the cutter then rotates, the periphery thereof will not track true, and the teeth will wander on the workpiece over a wider area than that intended.
These problems are exacerbated by periodic removal and replacement of cutters on the holder, in which a repeat of exact alignment is dubious.
Accordingly, what is needed is a mounting structure of cutters which solves the foregoing problems.